Evaporation of the disc by thermal conduction is a plausible mechanism
to form a truncated disc/ hot inner flow geometry
(e.g. [1]}). Counterintuitively, this is not most
efficient at the smallest radii, as these also have the highest
coronal densities, so have the highest condensation rates. Evaporation
first erodes a gap in the disc, leaving a residual inner
disc. This gap expands radially, eventually giving a fully truncated
disc if the mass accretion rate is low enough[2]}. However,
close to the transition, the residual disc can remain. This geometry
allows the outer disc to carry all the mass accretion rate. This disc
evaporates into a corona, truncating it at some large radius into a
coronal flow which carries all the mass accretion rate. However, as
the flow accretes to smaller radii, the increasing density gives an
increasing condensation rate and some small fraction of the material
can condense out of the hot flow. This forms a cool ring at the
innermost radii, with a mass accretion rate which is only a small
fraction of the total mass accretion rate, with the rest of the
material accreting via the coronal flow[3]}.
